No reaction —

RIP and good planet hunting, Kepler

A second reaction wheel failed on the exoplanet-hunting telescope

Very few experiments have changed the way we perceive our Universe, but the Kepler exoplanet survey telescope is one such example. Simply by monitoring a single patch of the sky continuously, it provided a new understanding of how many planets exist in the galaxy. Since its launch in 2009, Kepler identified 115 exoplanets with over 2,700 other potential planet candidates—including a number that are comparable in size to Earth or orbiting within the habitable zone where liquid water might exist.

However, Kepler is an orbiting telescope, unreachable by spacecraft for repairs. Today, NASA announced that a reaction wheel—required to keep the telescope pointed steadily in one direction—ceased functioning. This is the second reaction wheel failure, meaning Kepler can't continue to monitor the same stars and their exoplanets it has watched since 2009.

The Kepler engineering team had been anticipating this problem for some months, so this news was not unexpected. NASA associate administrator John Grunsfeld mentioned hope that engineers could restore communication with the control system managing the reaction wheel, but the fact that this state of failure has persisted for some days indicates how faint the hope is.

The telescope can continue to operate in "drift mode," where different portions of the sky could come into view, but it's uncertain how much science could be done in that configuration. Kepler has amassed a great deal of data still to be analyzed, though, so we'll continue to see new papers and announcements from its archives. Kepler will supply our exoplanet needs for some time to come.

Grunsfeld called the mission a resounding success, and few would disagree. Whether engineers get lucky and manage to restart the reaction wheel, Kepler stands as one of the most successful and inspiring of all astronomical endeavors. Kepler's successor, the Transiting Exoplanet Survey Satellite (TESS), is scheduled for a 2017 launch.

By far my favorite telescope to date. I hope they have a replacement or fix in the works, as Kepler seemed to be by far the most useful at "discovering" new information our local neighborhood and exciting the public's imagination and aspirations towards reaching the stars.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

Edit -- well despite all the down votes I'm glad I asked. I learned a lot from other commenters about the overall missions, its successes, failures, what we learned, the failure rates of satellites, why we couldn't service it, and what the cost meant in regards to its discoveries. Some of what I learned.

1. The mission is a success in that the satellite lasted as long as it was supposed to and came in within its stated budget.2. There was extra noise in the system due to the universe and the fact that other stars are more active than our own. This reduced the effectiveness of the satellite. So it would have been nice if it ran for a longer period of time to compensate for this.3. The system had one backup reaction wheel for a total of four. Unfortunately two failed. Having four is standard practice and it was crap luck that two failed because they can last for much longer.4. The Kepler satellite is really, really far from earth, which made servicing it out of the question.5. The Kepler satellite had to maintain its orientation in space, which meant moving parts, which meant it was far more likely to fail than something like Voyager is.

So it is great that the satellite met its mission goals, but when I think about it I honestly feel the designers must have been hoping it would last longer than it did. It's like when you buy something and it lasts exactly as long as the warranty (sort of).

Actually, no - you're thinking of a control moment gyro, which is actually a reaction wheel mounted INSIDE a (usually motorized) gyroscope. A reaction wheel itself is just a big flywheel, hard-mounted to coincide with one of the spacecraft's primary inertial axes. It's specifically intended to cancel out rotation along that axis alone.

A CMG, on the other hand, can actively and accurately change the orientation of a spacecraft, even when starting from a stable state with none of the CMGs spun up.

Kepler has done outstanding work, and as noted has still amassed a huge backlog of data that has yet to be analyzed. That should answer some of your question, mrtsherman. Also, it will still be able to generate data, just more brief impressions of areas its lens passes through rather then the detailed analysis of a single section of space that it has provided in the past.

With the replacement launching in a few years, we might just have cleared analysis of Kepler's backlog of already mapped data by the time TESS launches.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

Assuming that the now-retired shuttle program could have reached a high enough altitude to repair Kepler, this outcome is in part a side effect of cancelling the shuttle with no replacement.

It wouldn't be cost effective to service it. Kepler cost $550 million, how much do you think a servicing mission would cost, even if we had the shuttle and the shuttle could even get to it (it couldn't).

Honestly I'm dubious about the cost effectiveness of Hubble Servicing Missions also, but HST was more of NASA talisman than something to worry about the cost of.

The fact is, Kepler lived for it's minimum lifespan. We always hope these satellites will last longer (I myself work on one that has lived 6 years beyond it's required life), but when they do die like this, NASA will simply declare "Mission Successful" and move on.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

If Kepler needs three reaction wheels for precise pointing, why did it only have one spare? Why not two or three spare reaction wheels if the telescope is so important?

It has 4 reaction wheels, one had already failed but they were able to keep going. It's the failure of a second one that killed it. So yes, they already had 1 "spare". The reason why they don't have more is simply that more = more weight and cost, and more weight = more cost, so therefore the issue is cost!. You can't build huge amounts of redundancy into missions like this because they have to come in at a certain budget. Can you imagine providing double or triple redundancy for every system onboard?

Regarding the lifespan, the other thing to consider is the amount of data. In the time that it has been watching, it has come up with 27,000 candidates for planets. That alone is a huge number to confirm, and does show that the hypothesis was right, that planets are not rare.

If Kepler needs three reaction wheels for precise pointing, why did it only have one spare? Why not two or three spare reaction wheels if the telescope is so important?

I'd guess that they calculated an expected lifetime for the reaction wheels, and an expected lifetime for the project, and balanced these vs the cost of adding an extra spare.

With regards to the life time of kepler, if I had to wager a guess, Voyager probably doesn't have the same number of moving parts, and when you have systems like these the moving parts often break first. The reaction wheel is probably constantly spinning; given that, 4 years isn't bad at all.

Kepler was only *expected* to last 3.5 years at launch. Planck will last almost exactly 4 years. Not every space program is Voyager or Spirit.

Now, now, let's be precise: Kepler's Prime Mission was 3.5 years -- long enough to see 3 transits of a planet in an earth-like orbit around a sol-like star (i.e. with a 1-year orbit). This means it must last at least that long to be considered fully successful at accomplishing its mission. Not that it is only expected to last that long. If there was an actual expectation for the lifespan of the reaction wheels then it was certainly longer than 3.5 years -- you're not going to use a part that lasts on average exactly as long as you need when trying to guarantee that it lasts as long as you need.

What this also means is that Kepler was in fact completely successful at its Prime Mission. We're now in the Extended Mission, which was sadly cut short due to this failure. Extended missions are nice, but it's the Prime Mission that sets the benchmark for a project, and it's on the basis of being able to accomplish that mission that projects are authorized.

So basically, if 3.5 years of operation didn't seem worth $600 mil, then that would have been decided before Kepler ever flew. Based on the metric by which it was chosen, Kepler did its job and was worth it.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later?

No. Voyager doesn't have parts that need to move to keep it going, it just needs to stay pointed in the direction of earth (which isn't really moving from its perspective given its distance) and it can continue as long as the nuclear power source holds out. This telescope had continuously moving wheels that kept it pointed in the correct direction. Moving parts like that will always wear out.

Kepler was only *expected* to last 3.5 years at launch. Planck will last almost exactly 4 years. Not every space program is Voyager or Spirit.

Now, now, let's be precise: Kepler's Prime Mission was 3.5 years -- long enough to see 3 transits of a planet in an earth-like orbit around a sol-like star (i.e. with a 1-year orbit). This means it must last at least that long to be considered fully successful at accomplishing its mission. Not that it is only expected to last that long. If there was an actual expectation for the lifespan of the reaction wheels then it was certainly longer than 3.5 years -- you're not going to use a part that lasts on average exactly as long as you need when trying to guarantee that it lasts as long as you need.

What this also means is that Kepler was in fact completely successful at its Prime Mission. We're now in the Extended Mission, which was sadly cut short due to this failure. Extended missions are nice, but it's the Prime Mission that sets the benchmark for a project, and it's on the basis of being able to accomplish that mission that projects are authorized.

So basically, if 3.5 years of operation didn't seem worth $600 mil, then that would have been decided before Kepler ever flew. Based on the metric by which it was chosen, Kepler did its job and was worth it.

Yes and no. We exceeded the duration of the prime mission; but mostly fell short of that objective. One of Keplers early discoveries was that the average star has more/more active sun spots than ours. They injected enough unexpected noise into the data that the threshold for a detection was raised from 3 events to 5. As a result we'd've needed another 18 months for planets in Earthlike orbits around a Sunlike start to become candidates.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later?

No. Voyager doesn't have parts that need to move to keep it going, it just needs to stay pointed in the direction of earth (which isn't really moving from its perspective given its distance) and it can continue as long as the nuclear power source holds out. This telescope had continuously moving wheels that kept it pointed in the correct direction. Moving parts like that will always wear out.

Yes... but comm sats also use reaction wheels, frequently in a 4:3 redundancy and they are spec'd to +15 years operation. Having two wheels fail in 4 years? That's really short.

Yes... but comm sats also use reaction wheels, frequently in a 4:3 redundancy and they are spec'd to +15 years operation. Having two wheels fail in 4 years? That's really short.

Space operations still have a very low success rate compared to what we expect on earth. A not insignificant number of satellites never even make it to orbit because their rockets explode or fail in some other catastrophic way.

Yes and no. We exceeded the duration of the prime mission; but mostly fell short of that objective. One of Keplers early discoveries was that the average star has more/more active sun spots than ours. They injected enough unexpected noise into the data that the threshold for a detection was raised from 3 events to 5. As a result we'd've needed another 18 months for planets in Earthlike orbits around a Sunlike start to become candidates.

Good point; I'd forgotten about the unexpected sunspot noise.

I guess that's just the way of science -- you discover new things, which sometimes undermine the assumptions you were counting on to let you discover other new things.

Kepler was only *expected* to last 3.5 years at launch. Planck will last almost exactly 4 years. Not every space program is Voyager or Spirit.

Yes, the nominal mission was 3.5 years. No, it didn't meet is primary goal as per above, since Sun, the previosuly known star activity it was modeled after, turned out to be slightly calmer than the average star. So the Kepler team soon realized they needed the double amount of transits to reach the desired goal with the actual noise background.

Oh, it could have been worse, it turns out it is the planet systems that really differ in characteristics. But still, Kepler did just 1 of the 2 expected goals, establishing statistics over planets.

If Kepler needs three reaction wheels for precise pointing, why did it only have one spare? Why not two or three spare reaction wheels if the telescope is so important?

Because the nominal mission was projected to be about half of the one adjusted for the actual constraints.

They managed the nominal mission, but had bad luck. Reaction wheels can go for much longer, and 1 backup is standard. MGS had 4 wheels and was active 4 11 years with all 4 wheels (AFAIK). MRO has 4 wheels and is on its 7th year with all 4 wheels (AFAIK).

If not one of the targeted missions are lucky, then we have to wait for TESS, which means it will see nearby Earth analogs around Sun analogs, if we are very lucky, sometime 2020. Well, it will be just in time for some large telescopes that are able to characterize their atmospheres and look for signatures of life.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

I know this has been said in this thread already, but...Kepler and Voyager had vastly different missions, expected lifetimes, etc. Kepler's systems are vastly more sophisticated than Voyager's because it has to have a special cooling system, precision optics, extraordinarily precise control of orientation, etc. Kepler's designed lifetime was 3.5 years if I recall correctly, and it launched a little over 4 years ago. I'd say that's pretty good engineering (exceeding expected lifetime by a lot may mean you didn't know enough about the problem to make an accurate lifetime estimate)!

You shouldn't be looking at lifetime per dollar, you should be looking at science per dollar (not exactly an objective metric, but there you are). Kepler has produced far more data than Voyager, so it's arguable which was more cost efficient.

Granted I don't know much about satellite lifespans, but doesn't just over four years seem to be a remarkably short lifespan given Voyager is still operating thirty years later? According to Wikipedia the cost was $600million. Can't say I'm too impressed with the price to lifespan ratio. Please correct me if I'm off the mark here.

A bit off the mark. Four years is pretty good for a satellite telescope. Not fantastic, but not terrible either. I mean, Hubble was practically DOA.

Kepler was only *expected* to last 3.5 years at launch. Planck will last almost exactly 4 years. Not every space program is Voyager or Spirit.

And just to add even a bit more perspective: There's an article out today that shows that the New York Yankees currently have $85 million dollars worth of players on the injured reserve list right now. We have sports teams which have spent more than Kepler's entire budget on player salaries over the last 4 years and not won a damn thing to show for it.

While $600M is an awful lot of money to a person it is really quite "small change" when compared to the expenses of a country. We should be funding 3-4 of these types of projects every year instead of buying a couple hundred more tanks that the military people don't even want.